…and some of those are only observed at certain times of the year:
* CDT – Central Daylight Time
* CST – Cuba Standard Time
* EASST – Easter Island Summer Time
* EST - Eastern Standard Time

…it is unclear if the offset will change. For example, if the server was set to EST in the winter, then it would be EDT in the summer, or UTC-4.

Everything else in the Ubuntu world is set to UTC (including ubuntu-cdimage itself) because it gives us an unambiguous center regardless of where we are at in the world. It would only make sense that the tracker would follow suit.